Question

Consider two hosts \(P\) and \(Q\) connected through a router \(R\). The maximum transfer unit (MTU) of the link between \(P\) and \(R\) is 1500 bytes, and between \(R\) and \(Q\) is 820 bytes. A TCP segment of size 1400 bytes was transferred from \(P\) to \(Q\) through \(R\), with IP identification value as 0x1234. Assume that the IP header size is 20 bytes. Further, the packet is allowed to be fragmented. Which of the following statements is/are correct?

• A. Two fragments are created at \(R\) and the IP datagram size carrying the second fragment is 620 bytes.
• B. If the second fragment is lost, \(R\) will resend the fragment with the IP identification value 0x1234.
• C. If the second fragment is lost, \(P\) is required to resend the whole TCP segment.
• D. TCP destination port can be determined by analysing only the second fragment.

Hint:

IP fragmentation is handled at the network layer, but reliability and retransmission are responsibilities of TCP.

Answer 1

The Correct Option is A, C

Step 1: Fragmentation at router \(R\).
The original IP datagram size is: \[ 1400 \text{ (TCP data)} + 20 \text{ (IP header)} = 1420 \text{ bytes} \] The MTU between \(R\) and \(Q\) is 820 bytes, so fragmentation is required. Each fragment has a 20-byte IP header.
Maximum data per fragment: \[ 820 - 20 = 800 \text{ bytes} \] Thus: - First fragment data = 800 bytes \(\Rightarrow\) fragment size = 820 bytes
- Remaining data = \(1400 - 800 = 600\) bytes \(\Rightarrow\) fragment size = \(600 + 20 = 620\) bytes
Hence, option (A) is correct.

Step 2: Fragment loss handling.
IP is unreliable and routers do not retransmit lost fragments. Therefore, option (B) is incorrect.

Step 3: TCP-level retransmission.
If any fragment is lost, the entire IP datagram is discarded at the receiver. TCP detects loss and retransmits the entire TCP segment from the sender \(P\). Hence, option (C) is correct.

Step 4: TCP header analysis.
Only the first fragment contains the TCP header. The second fragment does not carry TCP port information, so option (D) is incorrect.

Step 5: Conclusion.
The correct statements are (A) and (C).

Final Answer: (A), (C)